1. Field of the Invention
The present invention relates to improved coating oil compositions for application to metal surfaces to protect the metal from rust and corrosion. More particularly, the coating oils of this invention are employed for the treatment of metal surfaces which are subsequently electrocoated and exhibit improved compatibility with the paint used in the electrodeposition process.
2. Description of the Prior Art
The need to protect the surface of metal sheet obtained from rolling operations, particularly the surfaces of ferrous metals, is recognized throughout the industry and a variety of coating oil compositions are available for this purpose. The coating oil is typically applied to the metal after final processing and before storage or shipment to protect the metal surface. It should also protect the metal from rust and corrosion during storage and shipment.
It is not uncommon for metal coils to be stored for prolonged periods prior to stamping and other metal working operations, often in hostile environments. For example, the coils may be subjected to a high humidity, wide temperature variations and acidic atmospheres from adjacent pickling baths, all of which promote rust and corrosion. The coating oil should prevent or at least minimize deterioration of the metal surface resulting from these conditions.
While the coating oil must be capable of forming a tenacious, continuous, hydrophobic barrier on the metal surface, it should not interfere with subsequent surface treatments, most notably, phosphatizing and painting. It must be capable of being readily and completely removed in subsequent annealing and/or washing operations so that it will not adversely affect adhesion or surface quality of the paint film. This is even more critical where the paint is applied by electrocoating.
Electrodeposition of paint, i.e. electrocoating, is commonly used for industrial painting of metal workpieces. The ability to deposit paint films in recessed areas coupled with the ability to use water-based paints and other advantages associated therewith have resulted in wide acceptance and use of electrodeposition processes by automobile and other manufacturers, notably by the appliance industries, to paint individual body parts and entire car bodies or other assemblies. For a general description of electrodeposition procedures, reference may be had to Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Ed., vol. 15, pp. 178-191 (John Wiley and Sons).
If coating oil residues are present on the metal workpiece being electrocoated, there residues can build up in the electrocoating dip tank and ultimately will result in surface imperfections in the cured paint film. These imperfections resemble small indents, streaks or other blemishes. A similar problem can occur if coating oils are trapped in areas such as seams, joints, etc., and can not be washed out prior to electrocoating. The trapped coating oil will be volatilized during the curing (baking) and can settle out on adjacent painted surfaces also causing formation of blemishes.
Accordingly, in formulating coating oils suitable for treating metal surfaces which are subsequently to be electrocoated, the formulations must not only prevent rust and corrosion but also must not interfere with the subsequent electrocoated film.
Numerous coating oils are known and reported in the literature to protect metal surfaces against rust and corrosion. Typically these products form a continuous, hydrophobic barrier on the metal surface which is impervious to moisture. Some are also effective against the corrosive action of acidic vapors. Most of these products comprise a carrier oil with a waxy and/or fatty substance and, optionally, one or more additives. U.S. Pat. No. 3,298,954, for example, describes compositions containing polybutenes, fatty acids, and in some cases sperm oil wax. The lubricant compositions of U.S. Pat. No. 3,600,310 which are disclosed for the protection of metal surfaces during coiling and uncoiling contain a mineral oil, a fatty acid or polymer or glyceride thereof, and a paraffin wax such as slack wax.
U.S. Pat. No. 4,166,151 discloses "waxy" ester compositions useful for protecting metal surfaces from acidic vapors which are derived from C.sub.10-25 aliphatic carboxylic acids and C.sub.15-40 aliphatic alcohols. These wax esters can be applied to the metal in an inert, volatile hydrocarbon. Coating oils which are blends of a petroleum sulfonate, a polymeric fatty acid and a hydrocarbon oil are described in U.S. Pat. No. 4,396,515. More specifically, the coating coil compositions are comprised at 1-15% polymeric fatty acid obtained from the polymerization of an unsaturated C.sub.16-20 monocarboxylic acid and containing at least 60% C.sub.36 dimer, 1-15% oil soluble Group IA alkali metal petroleum sulfonate, and 80-98% inert, substantially wax-free, naphthenic or paraffinic hydrocarbon oil. Slushing oil or rust inhibiting compositions comprised of a Group IIA, IIIA or IVA metal salt of a mahogany or naphthalene sulfonic acid, an aliphatic carboxylic acid, a cosolvent selected from aromatic hydrocarbons, methyl pyrrolidone, tetrahydrofuran and mono- and dialkyl ethers of alkylene glycols and mixtures thereof, and a paraffinic or naphthenic lubricating oil are disclosed in U.S. Pat. No. 3,875,789.
Whereas all of the formulations of the above references are effective for preventing corrosion of ferrous metals and in some instances are effective against the action of acidic vapors, they are all unsuitable for use where a metal workpiece formed from metal sheet coated with said oils is subsequently electrocoated. They all interfere to a greater or lesser extent with the electrodeposition process and produce undesirable and aesthetically unacceptable paint films.